3.5.4.1 Design Rules for Parts Made From Impression Die Forgings

Sections 3.5.4.1 through 3.5.4.5 give design rules that are specific to the designated forging process. Please refer to Section 2.5 Prints and Specifications for design information which is applicable to all forging processes.

1. All features should be oriented so that they can be formed in impressions moving in opposite directions, such as the part shown in Figure 3-9. Features such as undercuts and holes oriented other than in the direction of forging are not typically forged and must be fully machined. There are, however, a few special presses with side piercing capability, which permit forging of cross-oriented and hollow features.
   
   
   

   Figure 3-9 The most economical shape to forge is one that can be formed in impression moving in opposite directions.

2. Forging cost is minimized and tolerances reduced when forging loads are balanced, eliminating side loads on the machine members that restrain the dies. Figures 3.10, 3.11 and 3.12 shows an unbalanced condition and two die alternatives.
3. Sharp exterior corners require high forging pressures to fill the corresponding die features. Sharp interior corners (fillets) cause difficulties in metal flow, and may require one or more preform dies to attain, or may require additional machining operations. Therefore, radii should be as large as possible consistent with functional and assembly constraints. Corner and edge radii should also be uniform to minimize die sinking cost.
4. Interior corner (fillet) radii are dependent on forging severity (primarily rib height) and the forgeability of the alloy. Table 3-2 gives preferred and minimum fillet and corner radii for a 25 mm (1.0 in.) rib height, which corresponds to Part 2 in Figure 3-13.
   
   
   

   Figure 3-10 The forging as oriented generates a side thrust in the die requiring the counterlock to prevent lateral shift of the die. The counterlock is subject to wear from the side loads.

   Figure 3-11 The forging can be rotated in the die to balance the lateral loads and eliminate the counterlock. However, the holes in the bosses cannot be forged, and must be fully machined.

   
   
   

5. Draft angles should be the maximum allowable, consistent with functional, assembly and weight constraints. For ferrous forgings, draft angles less than 5^° usually prohibit the use of hammers. Dies installed in presses are usually equipped with knock-out pins to eject the forging from the cavity, and can produce forgings with little or no draft.

6. As a general rule, less draft is required on the outside of a feature than on the inside. (See Figure 3-14)
   
   
   

   Figure 3-12 Where production quantities justify two sets of impression dies, the forgings can be oriented opposite to balance the side loads. This arrangement permits the holes in the bosses to be forged to reduce the amount of machining required.

   Figure 3-13 This figure represent shapes that are progressively more difficult to forge.

7. Component features that are held to close tolerances should be formed in the same die member to avoid additional cross-die tolerance.

8. All datum targets and tooling points should be located on features made in the same die half, as illustrated in Figure 3-15. The upper die half is preferred since there is less contact between the die and the forging, and consequently less cooling.
9. See Appendix A Tolerances for Impression Die Forgings and Appendix D Specialized Tolerances for Precision Aluminum Forgings.

Table 3-2
Representative Fillet and Corner Radii for Forgings with 25 mm (1.0 in.) High Ribs